1). Field of the Invention
The present invention relates to a printed circuit board and to a card incorporating a printed circuit board wherein vias of the printed circuit board provide strength to the printed circuit board.
2). Discussion of Related Art
A card for a computer usually includes a printed circuit board with one or more electronic devices mounted thereto. A printed circuit board may, for example, comprise a substrate with a number of contact pads formed thereon and electronic devices, such as semiconductor dies which are mounted to package substrates, may be mounted to the printed circuit board via an array of solder balls which are attached to the contact pads.
A substrate of a printed circuit board usually consists of a number of insulative layers with networks of metal lines sandwiched between the insulative layers. The vias extend from the contact pads through the layers and are connected to one another by the network of metal lines.
It may be required that a card for a computer be used in either high temperature or low temperature environments. The printed circuit board and the electronic devices of the card are usually made of different materials having different coefficients of thermal expansion. Because of the mismatch in coefficients of thermal expansion, the printed circuit board and the electronic devices tend to expand and contract at different rates when the card is subjected to high or low temperatures. Differences in expansion rates of the printed circuit board and the electronic devices, in turn, translate to a buildup of stresses on the printed circuit board. Stresses that are created on the printed circuit board may cause contact pads of the printed circuit board to tear out of a substrate of the printed circuit board, or cause delamination of layers of the substrate. Tearing out of contact pads is particularly problematic in cases where a layer of the substrate in which or on which the contact pads are formed is made of a softer material such as may be required for photolithographic purposes.
One solution which may prevent damage to printed circuit boards of the above kind would be to reduce the size or surface area of the printed circuit board and the electronic devices. A reduction in size of a printed circuit board and the electronic devices may, however, require an increase in the number of layers of a substrate of such a printed circuit board and a substrate of such an electronic device, to allow for more metal lines to be formed so that signals can be properly routed to and from the electronic devices. Cost implications may limit the number of layers of a substrate of a printed circuit board or an electronic device of the above kind.